If you want to get that anal about extracting 8 cents of heat from
shower waste water, they do have heat recovery devices you could
install in the drain line. * It's basicly a heat exchanger that runs
the exiting waste water past the incoming cold water that is going to
your water heater. * I doubt it's practical, worth the cost/trouble,
etc but at least you don't have to stand in 4 inchs of water taking a
shower.

The drain line heat recovery device is what you need, like what's
described he

http://www.toolbase.org/Technology-I...-heat-recovery

Payback time in a retrofit situation is probably 100 years or
something, but it probably make sense from an environmental
sustainability point of view.

Ken